"This is a substantial study. There has never been such a summary over such a long time period. It is an enormous database," commented Dr Ursula Munro, an ornothological specialist at Australia's University of Technology, Sydney.

The birds were caught in mist-nets arranged in an alpine pass 1,920 m above sea level in Switzerland, during both day and night. No birds breed in the area, so all the birds caught were in transit. After recording their details the birds were released to continue their migration.

The researchers were surprised to find no uniform shifts in the timing of autumn migration, instead uncovering a complex adaptation which may constitute a serious threat to some species, in particular those with longer migratory flights.

Over the past two decades, spring temperatures in temperate regions have increased due to global warming, according to the team. This has brought forward the start of the reproductive season for many bird species. This is the first substantial study to look at an annual cycle of migratory birds like autumn migration.

The changes to migration seem to depend on how far the birds travel. Long-distance migrants, those that winter south of the Sahara, appear to be leaving earlier. But short distance migrants who winter in Southern Europe and Northern Africa have delayed their autumn migration.

One argument for why the long-distant migrants are leaving earlier is to avoid flying when it is really hot, Munro explained to ABC Science Online. "The earlier they go, the better chance they have of avoiding the dry period."

Migrating birds tend to fly at night and use the daytime for rest and feeding. The night is cooler and lessens the risk of predators. "They don't sleep much when they are migrating," said Munro.

Many birds die in the Sahara when they cross, suffer from the heat and can overheat very easily. Ironically, birds that are having a tough time tend to keep going, whereas birds in good condition may stay and 'refuel' for a while.

The suspicion that the change in climate has been affecting migration has been a long standing one, said Munro. She cites an example of related European birds where the European blackcap, which has a short migration, has been delaying its migrations. This means a similar bird, the European garden warbler, seems to be losing out because when it returns it must compete with the blackcap.

In another similar case, the long-distance migratory species is predicted to become extinct very soon. "The birds seem to be interbreeding, and now hybrids are out in the wild," she said. "We believe this is a phenomenon of genetic swamping, where one species is being absorbed into the gene pool of the other."

Overall the change in climate is problematic for birds with longer migrations. "It's a major problem," said Munro. "We don't know how climate change will affect the Sahara. Is the desert growing? The birds only have a certain capacity to get over it."